1. Field of the Invention
This invention relates to bonding metallic coatings to aluminum substrates. More particularly, the invention relates to a process for replacing the native aluminum surface oxides with stable coatings to promote a strong metallurgical/chemical bond with sprayed metal coatings.
2. Discussion of the Related Art
Aluminum and aluminum alloys are generally very reactive and rapidly form a passivating surface oxide film (5-100 manometers thick) when exposed to the atmosphere at ambient temperatures. Such oxide film inhibits adherence of metallic coatings to unroughened aluminum. Thus, to effect a metallurgical, chemical or intermetallic bond between the aluminum or aluminum alloy and other metals, it is often necessary to remove, dissolve or disrupt such oxide film. When so stripped of the oxide, aluminum or an aluminum alloy will readily bond with nickel, copper and iron based alloys at temperatures as low at 500.degree. C. Aluminum chemical etchants such as those described in U.S. Pat. No. 3,779,839, typically contain alkali metal fluorides, sodium acid fluoride and hydrogen fluoride; a chloride compound selected from NaCl and MgCl.sub.2 ; and Cr.sub.2 O.sub.3. Such techniques have proved disadvantageous either because of cost or because they are too disruptive of the substrate or the environment. In the absence of a commercially viable and environmentally clean methodology of removing native oxides from aluminum surfaces, roughening has heretofore been the principal means of bonding thermally spray coatings to cast aluminum surfaces. Such roughening has been carried out by mechanical means such as grit blasting, high pressure water, electric discharge machining or chemical etchants. It would be desirable if a method could be found that eliminated the need for roughening of cast aluminum substrates and yet enables the adherence of metallic coatings thereon.
Fluxes are readily used to remove the surface oxide films from aluminum. This is exemplified by the current commercial practice of brazing two pieces of aluminum alloy sheet metal (usually cold-rolled with a low temperature brazing metal layer) which are joined by first assembling the pieces in a jointed relationship and then flooding the joint area with a flux applied at room temperature. When heated aggressively, the flux melts and strips the surface oxides, thereby allowing the layer to form an interfacial alloy joint with the aluminum, as described in U.S. Pat. No. 4,911,351. The flux composition often has a fluoride or chloride base, as described in U.S. Pat. Nos. 3,667,111 and 5,318,764. Flux made of alkaloid aluminum fluoride or chloride salts have a melting temperature just below the melting temperature of aluminum alloys.
The use of flux has proved very effective when working with rolled aluminum sheet, but the flux will not work with cast aluminum alloys because cast aluminum is porous, non-homogenous, has no clad layer and melts at a temperature that overlaps the melting temperature of the fluxes. This is a significant drawback when (i) the metal that is to be bonded to the cast metal is a thermally sprayed and not the same as the cast metal, and (ii) the metal is applied as hot droplets without the presence of a low melting point braze metal.
Fluxless braze technology, such as presented in WO 97/36709 teaches the use of aluminum chemical etchants NaF, KF or HF in place of flux to improve the fillet forming capability of vacuum braze aluminum alloys. But, this reference required the presence of brazing materials between the articles to be joined.
Non-roughening thermal spray techniques include fluxing of the cast aluminum surface to remove surface oxide prior to thermally spraying coatings is the topic of U.S. Pat. No. 5,723,187. This reference discloses the steps of (1) depositing a flux material (i.e. potassium aluminum fluoride containing up to 50 molar percent other fluoride salts) onto such cast surface which has been cleansed to be substantially free of grease and oils, such deposition providing a dry flux coated surface, the flux being capable of removing oxide on the cast surface and having a melting temperature below that of the cast surface; (2) thermally activating the powder flux in the flux coated surface to melt and dissolve any oxide residing on the cast surface; and (3) concurrently therewith or subsequent to step (2) thermally spraying metallic droplets or particles onto the flux coated surface to form a metallic coating that is metallurgically bonded to the cast surface.
U.S. application Ser. No. 08/829,666 filed on Mar. 31, 1997, "Method of thermally spraying metallic coatings using flux cored wire" now U.S. Pat. No. 5,820,939, teaches a method that simultaneously apply the flux and the metallic coating unto cast aluminum surfaces using cored wire technology. It discloses the use of a cored wire for use in thermal spraying on aluminum alloy substrates having a powder core mixture consisting of (i) metal powder effective to metallurgically bond with the substrate when the metal powder is in molten condition, (ii) a fluxing powder effective to strip aluminum oxides from the substrate surface at appropriate temperatures, (iii) a pliable metal sheath encapsulating the powder mixture and having a composition that is compatible with said bonding metal and (iv) thermally spraying the said cored wire to produce a metallurgically bonded metal coating to the aluminum substrate.
U.S. Pat. No. 5,100,486 teaches a different process to apply flux to remove surface oxide and prepare the metal surface to receive and bond to the metal coating. The method consists of (i) forming a slurry with flux, the metal coating particles and an organic binder, (ii) applying the slurry to the metal substrate, (iii) heating to activate the flux, strip the surface oxide and evaporate the organic binder and (iv) furnace sintering to form a bond between the metal substrate and the metal coating layer.
In all of these non-roughening cases, a solid, commercially available and independent (of the aluminum substrate) flux powder is used to dissolve the substrate surface oxide prior to or concurrently with coating bonding. Advantageously, the current invention teaches the use of the aluminum alloy substrate to grow the flux crystals prior to thermal spraying operation.
While the current invention deposits a coating similar in composition to fluoride fluxes, the inventive double fluoride composition behaves differently than conventional flux. While not wishing to be bound to the following theory, it is believed that the aqueous KF solution reacts with the native aluminum oxide and at proper concentration forms a protective coating layer of a double potassium aluminum fluoride salt which inhibits oxide regrowth.
Therefore, the primary object of this invention is to achieve a method that economically, reliably and instantly bonds thermally sprayed metallic droplets or particles onto an unroughened cast light metal-based substrate without the presence of conventional brazing materials. The method should provide a metallurgical and/or chemical bond between such light metal and thermally sprayed metallic coatings should involve no application of any powdered flux materials as practices by the prior art. The process is also advantageous for manufacturing in that, (1) an aqueous bath replaces the costly fluxing operation, and (2) it eliminates powder handling and thereby more environmental friendly.